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A pixel (a contraction of picture element) is one of the many tiny dots that make up the representation of a picture in a computer's memory. Usually the dots are so small and so numerous that, when printed on paper or displayed on a computer monitor, they appear to merge into a smooth image. The colour and intensity of each dot is chosen individually by the computer to represent a small area of the picture.
The more pixels used to represent an image, the closer the result will resemble the original. The number of pixels in an image is called the resolution. This can be expressed as a single number, as in a 'three megapixel' digital camera, which has three million pixels, or as a pair of numbers, as in a '640 by 480 display', which has 640 pixels from side to side and 480 from top to bottom (as in a VGA display), and therefore has a total number of 640 × 480 = 307,200 pixels.
The coloured dots that form a digitized image (such as a JPG file used on a web page) are also called pixels. Depending on how a computer displays an image, these may not be in one-to-one correspondence with screen pixels. In areas where the distinction is important, the dots in the image file may be called texels.
In computer programming, an image composed of pixels is known as a bitmapped image or a raster image. The word raster originates from analogue television technology. Bitmapped images are used to encode digital video and to produce computer-generated art.
Since the resolution of the computer display can be adjusted from the computer's operating system, a pixel is a purely relative measurement. The modern computer display is designed with a native resolution which refers to the perfect match between pixels and triads. The native resolution will produce the sharpest picture capable from the display. However since the user can adjust the resolution, the monitor must be capable of displaying the resolution, which is accomplished by drawing each pixel out of more than one triad. This process usually results in a fuzzy picture. For example, a display with a native resolution of 1280x1024 will look best set at 1280x1024 resolution, will display 800x600 adequately by drawing each pixel with more physical triads, and will be unable to display in 1600x1200 at all due to the lack of physical triads.
Usually a non-native resolution is better displayed on a CRT than on an LCD.
Pixels are either rectangular or square. A number called the aspect ratio describes the squareness of a pixel. For example, a 1.25:1 aspect ratio means that each pixel is 1.25 times wider than it is high. Pixels on computer monitors are usually square, but pixels used in digital video have non-square shapes, such as the D1 aspect ratio.
Each pixel in a monochrome image has its own brightness. Zero usually represents black, and the maximum value possible represents white. For example, in an eight-bit image, the maximum unsigned value that can be stored by eight bits is 255, so this is the value used for white.
In a colour image, each pixel has its own brightness and colour, usually represented as a triplet of red, green and blue intensities (see RGB). Full-colour LCD flat panels and CRT monitors use pixels made of 3 sub-pixels.
The number of distinct colours that can be represented by a pixel depends on the number of bits per pixel (BPP). Common values are:
8 bpp (256 colours)
16 bpp (65,536 colours, known as Highcolour)
24 bpp (16,777,216 colours, known as Truecolour).
Images composed of 256 colours or fewer are usually stored in the computer's video memory in chunky or planar format, where a pixel in memory is an index into a list of colours called a palette. These modes are therefore sometimes called indexed modes. While only 256 colours are displayed at once, those 256 colours are picked from a much larger palette, typically of 16 million colours. Changing the values in the palette permits a kind of animation effect. The animated startup logo of Windows 95 and Windows 98 is probably the best-known example of this kind of animation.
For depths larger than 8 bits, the number is the total of the three RGB (red, green and blue) components. A 16-bit depth is usually divided into five bits for each of red and blue, and six bits for green (green gets more bits because the eye is more sensitive to that colour). A 24-bit depth allows 8 bits per component. On some systems, 32-bit depth is available: this means that each 24-bit pixel has an extra 8 bits to describe its opacity. On older systems, 4 bpp (16 colours) is also common.
When an image file is displayed on a screen, the number of bits per pixel is expressed separately for the raster file and for the display. Some raster file formats have a greater bit-depth capability than others. The GIF format, for example, has a maximum depth of 8 bits, while TIFF files can handle 48-bit pixels. There are no displays that can display 48 bits of colour, so this depth is typically used for specialized professional applications with film scanners and printers. Such files are rendered on a screen with 24-bit depth.
Other objects derived from the pixel, such as the voxel (volume element), texel (texture element) and surfel (surface element), have been created for other computer graphics uses.
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